docs: def import_csv(self, file_name='*', folder_name='.', head_row=0, index_col=0, convert_col=True, concat_files=False):
          """ Imports csv file(s) and stores the result in data.
              
          Note
          ----
          1. If folder exists out of current directory, folder_name should contain correct regex
          2. Assuming there's no file called "\*.csv"
  
          Parameters
          ----------
          file_name       : str
              CSV file to be imported. Defaults to '\*', i.e. all csv files in the folder.
          folder_name     : str
              Folder where file resides. Defaults to '.', i.e. current directory.
          head_row        : int
              Skips all rows from 0 to head_row-1
          index_col       : int
              Skips all columns from 0 to index_col-1
          convert_col     : bool
              Convert columns to numeric type
          concat_files    : bool
              Appends data from files to result dataframe
  
          """
  
          # Import a specific or all csv files in folder
          if isinstance(file_name, str) and isinstance(folder_name, str):
              try:
                  self.data = self._load_csv(file_name, folder_name, head_row, index_col, convert_col, concat_files)
              except Exception as e:
                  raise e
  
          # Import multiple csv files in a particular folder.
          elif isinstance(file_name, list) and isinstance(folder_name, str):
  
              for i, file in enumerate(file_name):
                  if isinstance(head_row, list):
                      _head_row = head_row[i]
                  else:
                      _head_row = head_row
  
                  if isinstance(index_col, list):
                      _index_col = index_col[i]
                  else:
                      _index_col = index_col
  
                  try:
                      data_tmp = self._load_csv(file, folder_name, _head_row, _index_col, convert_col, concat_files)
                      if concat_files:
                          self.data = self.data.append(data_tmp, ignore_index=False, verify_integrity=False)
                      else:
                          self.data = self.data.join(data_tmp, how="outer")
                  except Exception as e:
                      raise e
  
          else:
              # Current implementation can't accept,
              # 1. file_name of type str and folder_name of type list(str)
              # 2. file_name and folder_name both of type list(str)
              raise NotImplementedError("Filename and Folder name can't both be of type list.")